The present invention relates to the preparation of flexible, optically transparent and electrically conductive coatings and layers based on carbon nanotubes.
Carbon nanotubes (CNTs) are a class of nano-materials that have the potential to provide a variety of new, and previously unattainable, combinations of properties to materials. One emerging area for the use of carbon nanotubes is solution-processed, flexible and durable, conductive coatings. Carbon nanotubes can be dispersed in a medium and processed by conventional solution processing methods to yield flexible/durable, conductive layers. Such processes, if desired, may also produce transparent, flexible/durable, conductive layers. Another emerging area for using carbon nanotubes is in the fabrication of conductive polymer composites. Carbon nanotubes can be dispersed in a polymer matrix to yield a conductive polymer composite that retains the mechanical and processing properties of the polymer.
Because of their high electrical conductivity, nanometer diameters, high aspect ratios, and high degree of flexibility, carbon nanotubes are ideal materials for the preparation of transparent conductive films and coatings. Starting from highly dispersed nanotube inks, nanotube network films can be prepared on flexible or rigid substrates by various solution processing methods. Flexible transparent conductors prepared from single walled (SWNTs) and multi-walled (MWNTs) carbon nanotubes have been reported. SWNTs have smaller diameters than MWNTs (approx. 1 nm versus 30 nm) and may produce coatings having more desired qualities.
Currently, conductive coating of carbon nanotubes have been produced and used as antistatic coatings and for electromagnetic shielding. However, additional potential uses for conductive coatings using carbon nanotubes include touch screens for computers and other video terminals, flat panel displays, and as a substitute for expensive indium tin oxide coatings. There remains a need in the art to provide carbon nanotube coatings having both higher degrees of optical transmission in combination with higher electrical conductivities.
One approach that shows promise in improving both the electrical conductivity and optical transmission of carbon nanotube coatings is by adding chemical reactants that p-dope or n-dope the carbon nanotubes, A variety of p-type and n-type dopants have been explored, including Br2, I2, and O2 as p-type and K, Cs, and Na as n-type.
Thionyl chloride has been shown to yield substantial improvements in the conductivity of SWNTs. (Skakalova et al J Phys Chem B 2005, 109, 7174 and Dettlaff-Weglikowska et al J. Am. Chem. Soc. 2005, 127, 5125). Thionyl chloride has benefits compared to prior art doping agents such as Br2, Cs, and K, in that it is substantially less reactive and easier to handle. Thionyl chloride is used commercially in lithium-thionyl chloride batteries. Roth and co-workers showed that SWNT powders or bucky papers treated with liquid thionyl chloride for 24 h at 45° C. showed conductivity increase by up to a factor of 5. However, their results indicate that the treatment is less effective for larger diameter SWNT, such as those obtained by arc discharge and laser ablation processes. In addition, the reaction time is prohibitively long for use in-line during processing of carbon nanotube coatings.
There remains a need for doping processes that are suitable for a variety of CNT structures, fast, and yield stable doped structures.